One known process of joining a plurality of metal sheets together is a resistance welding process in which the metal sheets are stacked in a stacked assembly. Then, after the stacked assembly is gripped and pressed by a set of welding electrodes, an electric current is passed between the welding electrodes to melt a region of the metal sheets near contact surfaces thereof. When solidified, the melted region is turned into a solid phase, which is referred to as a nugget. In certain cases, three or more metal sheets are joined together by the resistance welding process.
Thicknesses of metal sheets or workpieces to be resistance-welded may not necessarily be identical to each other, but differ from each other in most cases. Therefore, the metal sheets tend to include a workpiece, the thickness of which is the smallest (hereinafter also referred to as a “thinnest workpiece”).
If the stacked assembly of metal sheets, including the thinnest workpiece disposed on an outermost side of the stacked assembly, is resistance-welded while pressing forces, which are applied to the stacked assembly by a pair of respective welding electrodes, are held in substantial equilibrium with each other, then the nugget that is formed between the thinnest workpiece and the workpiece adjacent thereto may not grow sufficiently. The reason for insufficient growth of the nugget is considered to be based on the fact that, if the stacked assembly comprises three stacked workpieces, then since the contact resistance between the thinnest workpiece and the adjacent workpiece is reduced due to ends of the thinnest workpiece flexing away from the adjacent workpiece, a sufficient amount of Joule heat is not generated between the thinnest workpiece and the adjacent workpiece, as disclosed in Japanese Patent No. 3894545.
The electric current, which is passed between the welding electrodes, may be increased in order to cause the nugget near the thinnest workpiece to grow sufficiently, thereby increasing the amount of Joule heat generated by the thinnest workpiece. However, such an increased amount of electric current tends to flow into the thicker workpieces, unduly melting the workpieces and producing scattered metal particles, thereby resulting in sputtering.
It may be considered effective to increase the energization time over which electric current is passed between the welding electrodes. However, it is not easy for the thinnest workpiece to be made to generate a sufficient amount of Joule heat, even with an increased energization time. The increased energization time also leads to a reduction in welding efficiency because the welding time is increased.
The present applicant has proposed in Japanese Patent No. 3894545 that the pressing force applied by the welding electrode, which is pressed against the thinnest workpiece, should be made smaller than the pressing force applied by the other welding electrode. By adjusting the pressing forces applied by the welding electrodes to the stacked assembly in this manner, it is possible to bring the amount of Joule heat generated at the interface between the workpieces into substantial equilibrium. Consequently, it is possible to allow the nugget between the thinnest workpiece and the adjacent workpiece to grow to a size which is substantially the same as the size of the nugget between the adjacent workpiece and the other workpiece.